In the course of the project thermal emission spectroscopy was used to study nonlinearities of quasi-particles which are generated in the strong-coupling regime, so-called polaritons. The previously conducted emission studies were extended from the solid to the liquid phase. Technical development of the method led to the conclusion that polariton nonlinearities known from electronic strong coupling could not be identified in the studied vibrational systems by means of the thermal emission approach. Second, the off-resonant Kerr nonlinearity of the neat liquid CS2 was investigated under strong coupling. Optical Kerr gating was used, that is, a strong pump light pulse induces polarization changes of a weak probe pulse by means of the molecular chi(3) nonlinearity. CS2 is both known for its strong Kerr-effect and its signatures of the ultrastong coupling regime if the vibrational transition around 1510 cm-1 is in resonance with a cavity mode. The ultrastrong coupling regime leads to a ground state energy shift, and thus it is expected to significantly influence the molecule’s nonlinear properties. Kerr-gating experiments were performed with pump beams at wavelengths between 700 nm and 1800 nm and probe beams at 800 nm. Fabry-Perot cavities were studied, either with metallic mirrors (gold, silver) or distributed Bragg reflectors. A long-lived strong nonlinearity was observed, that however originated from residual mirror absorptions and the light interaction with quasi-free carriers. No change in the nonlinear refractive index of CS2 was detected.
Both studies are expected to benefit from an all mid-infrared pump-probe setup. First, higher polariton populations than in the thermal emission studies could be achieved by laser pumping. Second, the Kerr effect of CS2 could be investigated near resonance where the impact of strong coupling has already been proven for other nonlinearities. A commercial Ti:sapphire amplifier – TOPAS light converter system was used as the basis for the study. Herriot-cell and multi-plate spectral broadening of the TOPAS idler beam at 1900 nm were studied. Best results were obtained with the multi-plate approach, promising self-compressed sub-20 fs pulse with octave-spanning pulses. The multi-plate continuum was used to seed the mid-infrared generation in pump- and probe-arm. Ultrashort pulses tunable from 3.5 to > 14 microns were demonstrated, constituting an ideal basis for time-resolved pump-probe spectroscopy in the mid-infrared.
Overview in respect of initially specified working packages (WP):
• WP1: Broadly tuneable ultrafast mid-IR pump-probe setup was developed. Publication in peer-reviewed journal is in preparation.
• WP2: Optical Kerr-gating was performed at the neat liquid CS2 under vibrational ultrastrong coupling. Cavity design led to elimination of parasitic nonlinearities from free carriers generated in the Fabry-Perot cavity mirrors. Afterwards, coupled and uncoupled nonlinear responses of CS2 were comparable.
• WP3: Thermal emission studies were conducted to follow up on earlier indication of polariton nonlinearities. By means of technical development, earlier results could not be confirmed which led to a short journal publication. Another manuscript documenting the performed experiments was sent to peer-review. Work has been presented on an international conference and publication of a revised version is anticipated.
